U. S. B. F.— Doc. 923. 



Fig. U.— Rock pool (M 2) on shore of Cedar Lake, the lake waters to the left foreground, a library 

 card floating on the surface of the pool and a tree root and rock separating the two. 



m-.vififr^'if^mia Lji|(.f«^i^« 



Fig. 12.— The inlet of the larger pond in the University of Pennsylvania botanical garden showing 

 one of the screen check pens (A 6). 



